The 3-Hour Oral Glucose Tolerance Test (OGTT) is a diagnostic procedure administered to pregnant individuals who have had an elevated result on the initial one-hour glucose screening test. This follow-up examination is performed to determine if the body is effectively managing sugar levels during pregnancy, a condition known as Gestational Diabetes Mellitus (GDM). The test requires a specific, controlled intake of sugar followed by timed blood draws to measure the body’s response. The entire process provides a detailed profile of glucose processing, moving beyond the simple screening result to establish a definitive diagnosis.
The Specific Glucose Load
The amount of sugar in the diagnostic drink for the three-hour test is a precise and concentrated dose of 100 grams of glucose, specifically dextrose. This quantity is substantially higher than the 50-gram load used in the initial screening test. To put this 100-gram dose into perspective, it is roughly equivalent to the total sugar content found in about 2.5 standard 12-ounce cans of regular soda. The commercially prepared solution, often called Glucola, contains this dextrose, water, citric acid, and flavoring. Dextrose is chosen because it is rapidly absorbed and directly reflects the body’s ability to handle a pure glucose spike. The concentrated nature of the drink is designed to create a maximal metabolic challenge.
Purpose of the Three-Hour Test
The high 100-gram glucose load is necessary because the test is designed to “stress test” the body’s ability to regulate blood sugar, effectively pushing the system to its limit. Pregnancy hormones, such as human placental lactogen (HPL), naturally increase insulin resistance, making it harder for the body’s cells to use insulin efficiently. This resistance is a normal physiological change in late pregnancy, but sometimes the pancreas cannot produce enough insulin to overcome it.
The goal is to measure the full capacity and timing of the body’s insulin response over several hours. The three-hour test confirms a diagnosis of GDM by mapping the entire metabolic curve. If the body fails to bring the blood sugar level back down to acceptable ranges at the one, two, or three-hour marks, it indicates that the insulin response is insufficient or delayed.
The Three-Hour Testing Procedure
The three-hour test requires careful preparation to ensure the results are accurate. Before the test, you must fast for at least eight to twelve hours, consuming only plain water. For at least three days leading up to the test, you are instructed to maintain a diet that includes at least 150 grams of carbohydrates daily to avoid falsely elevated results.
The procedure begins with a baseline blood draw to measure your fasting blood glucose level. Following this initial draw, you must consume the entire 100-gram glucose solution within five minutes. The time you finish the drink is carefully noted, as it marks the beginning of the timed portion of the test. After consuming the drink, you must remain at the testing site, seated, for the full three hours, as any physical activity can interfere with the results. Blood samples are then drawn precisely at the one-hour, two-hour, and three-hour marks.
What the Results Mean
The three-hour OGTT generates four separate blood sugar values: one fasting value and three post-glucose values. Each of these values is compared to established threshold levels, such as those recommended by the American College of Obstetricians and Gynecologists (ACOG). A diagnosis of Gestational Diabetes Mellitus is confirmed if two or more of the four blood sugar values meet or exceed the specific threshold for that time point.
Threshold values are often set as follows:
- Fasting sample: 95 mg/dL
- One hour: 180 mg/dL
- Two hours: 155 mg/dL
- Three hours: 140 mg/dL
If the diagnosis is positive, the next steps involve working with a healthcare provider and a dietitian to implement dietary modifications and begin monitoring blood sugar levels regularly.